The present invention relates to an image forming method, an image forming apparatus, a photoconductor and a process cartridge using the photoconductor.
An image forming process employing coherent light such as laser beam for writing light is widely used in a field of electrophotography for forming a digital image, for example in copying machines, printers and facsimiles. In such a process, there tends to arise a problem of occurrence of interference fringes in a formed image due to interference of coherent light in a photoconductor.
It is known that when the photoconductor meets with the following relation:
2nd=mxcex
(wherein n is a reflective index of a charge transporting layer, d is a thickness of the charge transporting layer, xcex is a wavelength of the writing light and m is an integer), the writing light is enhanced to cause interference fringes.
For example, when xcex=780 nm and n=2.0, a set of interference fringes is generated every time the thickness of the charge transporting layer is changed by 0.195 xcexcm. In order to eliminate such interference fringes completely, therefore, the charge transporting layer should have a thickness variation not greater than 0.195 xcexcm all over the image forming area. However, it is very difficult to produce such a photoconductor for an economical reason. Thus, various methods for restraining interference fringes have been proposed.
For example, Japanese Laid-Open Patent Publication No. S57-165845 discloses a photoconductor having a charge generating layer containing amorphous Si, wherein a light absorbing layer is provided on a surface of an aluminum support to prevent mirror reflection of light on the surface of the support, thereby preventing occurrence of interference fringes. This method is effective to a photoconductor having a layer structure consisting of an aluminum support/a charge transporting layer/a charge generating layer such as an amorphous Si photoconductor but is not very effective to a photoconductor having a layer structure consisting of an aluminum support/a charge generating layer/a charge transporting layer as seen in many organic photoconductors.
Japanese Laid-Open Patent Publication No. H07-295269 discloses a photoconductor having a layer structure consisting of an aluminum support/an under coat layer/a charge generating layer/a charge transporting layer, wherein a light absorbing layer is provided on the aluminum support to prevent interference fringes. However, even with this photoconductor, it is impossible to prevent interference fringes completely.
Japanese Examined Patent Publication No. H07-27262 discloses an image forming apparatus having a photoconductor including a cylindrical support having a convex shape obtained by superimposing a sub-peak on a main peak in a cross-section cut along a plane including the central axis thereof, and an optical system for irradiating coherent light with a diameter smaller than one cycle of the main peak to the photoconductor. The image forming apparatus can restrain interference fringes to a large extent with some limited types of photoconductors. However, many of photoconductors including a support having a convex shape obtained by superimposing a sub-peak on a main peak in a cross-section cut along a plane including the central axis thereof still generate interference fringes.
A photoconductor in which surface roughness of a support, an intermediate layer and/or an outermost layer is specified is known. For example, Japanese Laid-Open Patent Publication No. H10-301311 discloses a photoconductor including a photoconductive layer supported on a support, wherein the center-line surface roughness Ry of the support is at least xc2xd of the wavelength of the writing light beam. The photoconductor can reduce interference fringes when used in an image forming apparatus having a low resolution. However, when the spot diameter of the writing light beam is reduced so as to improve the resolution, interference fringes are unavoidably formed. The surface roughness Ra can properly represent magnitude of average unevenness of a sectional curve composed of only waves with similar amplitudes. However, an actual sectional curve of a photoconductor is composed of a multiplicity of waves of greatly different wavelengths and amplitudes. Minute waves superimposed on waves with large amplitudes are cancelled in calculating Ra and thus are not reflected in Ra at all.
Japanese Laid-Open Patent Publication No. H6-138685 discloses a photoconductor including a conductive support having a ten-point surface roughness Rz of 0.01-0.5 xcexcm and a surface protective layer having an Rz of 0.2-1.2 xcexcm. However, a surface protective layer is generally poor in hole transferring ability so that the photoconductor tends to cause a problem of an increase in electric potential of a latent image and to produce an unclear image by influences of ion species generated by electrification, oxidizing or reducing gas, humidity and so on. Also, it is extremely difficult to specify an Rz to eliminate interference fringes completely. When the writing light of the image forming apparatus has a high resolution, image defects such as interference fringes tend to occur.
Japanese Laid-Open Patent Publication No. H7-13379 discloses a photoconductor including an intermediate layer having and a surface protective layer for the purpose of preventing interference fringes such as moire. Further, for the purpose of preventing white spots in a solid pattern, the intermediate layer and the surface protective layer have specific ten-point surface roughness Rz of not greater than 1.0 xcexcm. However, the Rz for each layer is not disclosed to be effective to prevent interference fringes such as moire.
Japanese Laid-Open Patent Publication No. H08-248663 discloses a photoconductor including a support having a surface roughness of 0.01 to 2.0 xcexcm, and an outermost layer having a surface roughness of 0.1 to 0.5 xcexcm and containing inorganic particles having an average particle diameter of 0.05-0.5 xcexcm. However, it is not specified what kind of surface roughness is the surface roughness of the support and the outermost layer.
Conventional parameters of surface roughness include Rmax, Rz and Ra. It is well known that measured surface roughness values are largely varied depending upon the parameters adopted and upon the measurement conditions such as measurement length. Moreover, even with the same photoconductor, the degree of interference fringes vary depending upon the resolution of the image forming device, the wavelength of the writing light, the spot diameter of the writing light, etc. Thus, with the known techniques, it is impossible to produce images free of interference fringes while retaining other desired image qualities. It is also necessary to design, with a try-error technique, a desired photoconductor suited for a specific image forming device.
It is, therefore, an object of the present invention to provide an image forming method which has overcome the problems of the prior arts.
Another object of the present invention is to provide an image forming method of the above-mentioned type which is capable of producing high-quality images free from image defects such as interference fringes, streaks, and light and shade spots.
It is a further object of the present invention to provide an image forming method capable of producing a high-quality image free from image defects without lowering the resolution of an output image.
It is a further object of the present invention to provide an image forming method which permits the use of cheap support of a photoconductor.
It is a further object of the present invention to provide an image forming method capable of producing a high-quality image free from image defects at a high image forming speed.
It is a further object of the present invention to provide an image forming method capable of producing an image which is natural like a photo.
It is a further object of the present invention to provide an image forming apparatus capable of producing a high-quality image free from image defects without lowering the resolution of an output image.
It is a further object of the present invention to provide an image forming apparatus capable of forming an image on any desired image receiving medium.
It is a further object of the present invention to provide an image forming apparatus capable of forming a high quality image free of defects such as image missing, image scattering, blurs and interference fringes.
It is yet a further object of the present invention to provide a photoconductor capable of producing a high-quality image free from image defects such as interference fringes.
It is a further object of the present invention to provide a process cartridge having mounted thereon the above photoconductor which permits easy maintenance and exchange of parts contained therein.
In accordance with one aspect of the present invention, there is provided an image forming method comprising scanning a surface of a photoconductor having a photoconductive layer provided on a conductive support at least along a main scanning direction of said photoconductor with a writing light beam having a wavelength of xcex xcexcm and a spot diameter xcfx86 xcexcm to form an electrostatic latent image thereon, wherein the surface of said photoconductor has such a roughness that the sectional curve thereof along said main scanning direction is represented by a function y=f(x) when the main scanning direction and the direction of the vertical height are assumed to be the X-axis and the Y-axis, respectively, and wherein the maximum height of said sectional curve in a region from an arbitral position x on the X-axis to a position (x+xcfx86) on the X-axis is at least xcex/2n xcexcm where n is a refractive index of said photoconductive layer at the wavelength of said light beam, said maximum height being a distance between the maximum level in the direction of the Y-axis of the sectional curve in said region and the minimum level in the direction of the Y-axis of the sectional curve in said region.
In another aspect, the present invention provides an image forming apparatus comprising a photoconductor having a photoconductive layer provided on a conductive support, and an exposing means for scanning a surface of the photoconductor with a writing light beam having a wavelength of xcex xcexcm and a spot diameter xcfx86 xcexcm at least along a main scanning direction to form an electrostatic latent image thereon, wherein the surface of said photoconductor has such a roughness that a sectional curve thereof along said main scanning direction is represented by a function y=f(x) when the main scanning direction and the direction of the vertical height are assumed to be the X-axis and the Y-axis, respectively, and wherein the maximum height of said sectional curve in a region from an arbitral position x on the X-axis to a position (x+xcfx86) on the X-axis is at least xcex/2n xcexcm where n is a refractive index of said photoconductive layer at the wavelength for said light beam, said maximum height being a distance between the maximum level in the direction of the Y-axis of the sectional curve in said region and the minimum level in the direction of the Y-axis of the sectional curve in said region.
The present invention also provides a photoconductor adapted to be scanned with a writing light beam having a wavelength of xcex xcexcm and a spot diameter xcfx86 xcexcm at least along a main scanning direction to form an electrostatic latent image thereon, comprising a photoconductive layer provided on a conductive support, wherein the surface of said photoconductor has such a roughness that the sectional curve thereof along the main scanning direction is represented by a function y=f(x) when the main scanning direction and the direction of the vertical height are assumed to be the X-axis and the Y-axis, respectively, and wherein the maximum height of said sectional curve in a region from an arbitral position x on the X-axis to a position (x+xcfx86) on the X-axis is at least xcex/2n xcexcm where n is a refractive index of said photoconductive layer at the wavelength of said light beam, said maximum height being a distance between the maximum level in the direction of the Y-axis of the sectional curve in said region and the minimum level in the direction of the Y-axis of the sectional curve in said region.
The present invention further provides a process cartridge freely detachable from an image forming apparatus, comprising the above photoconductor, and at least one means selected from the group consisting of charging means, image exposure means, developing means, image transfer means, and cleaning means.
Interference fringes of an image formed by electrophotography employing a coherent light are considered to be attributed to differences in image density among pixels caused as a result of multiple reflection of the light in the photoconductor surface. It has been found that when such multiple reflection occurs in each pixel, the image density level of the image as a whole varies uniformly, and that when the pixel is sufficiently small, interference fringes in each pixel are hardly recognized with the naked eyes. Namely, when very minute interference fringes invisible with naked eyes are positively formed, the interference fringes have been found not to be visually recognized as a whole. It has also been found that interference fringes may be prevented when minute unevenness of specific characters is provided on a surface of a photoconductor.
When an electrostatic latent image is formed on a surface of the photoconductor by irradiation with a light having a specific light spot diameter and when an arbitral region of the surface having a diameter equal to the light spot diameter has a portion in which the writing light strength is intensified by an light interference therewithin but has no portion in which the writing light strength is weakened by light interference, then the average intensity of the writing light in that region is greater as compared with the case in which no light interference occurs. Namely, the image density corresponding to that portion (pixel) is relatively high. Conversely, when an arbitral region of the surface having a diameter equal to the light spot diameter has a portion in which the writing light strength is reduced by an light interference therewithin but has no portion in which the writing light strength is increased by light interference, then the average intensity of the writing light in that region is lower as compared with the case in which no light interference occurs. Namely, the image density corresponding to that region (pixel) is relatively low. Such a variation of the image density will cause image defects such as interference fringes.
On the other hand, when an arbitral region of the surface of the photoconductor having a diameter equal to the light spot diameter has a portion in which the writing light strength is intensified by an light interference therewithin and another portion in which the writing light strength is reduced by an light interference therewithin, then the average intensity of the writing light in that region is similar to the case in which no light interference occurs. Namely, the image density corresponding to that region (pixel) is nearly equal to the ordinary case. Since the area of one pixel is too small to be recognized by the naked eyes, no image defects such as interference fringes occur.